As known, modern aircraft's are equipped with a plurality of electrical and electronic devices requiring an input of electrical power to fulfil their functions. Such devices are environmental controls and systems, as well as, especially in passenger aircraft's, outlets for passenger's use and comfort. Whilst electrical power is often provided by power generators driven by the engines of the aircraft, passenger aircraft's are nowadays aircraft's generally equipped with auxiliary power units (‘APU’) as an alternative, supplementary or additional source of power for electricity needed on the aircraft. APUs are used to supply electrical power to the aircraft's systems and components while the aircraft is parked on the ground, in flight and during taxiing or landing.
An APU is a gas turbine engine mounted in the tailcone or in the wheelwell of the aircraft, i.e. inside the aircraft so that an intake duct is needed to feed outside air to the APU, as well as an outlet duct through which exhaust gases are expelled from the APU. Typically, an APU comprises a compressor inlet (‘plenum chamber’), a compressor, a combustor having a primary fuel nozzle and a secondary fuel nozzle, a turbine, a gas exhaust, and a shaft on which the compressor and the combustor are mounted for rotation on a shaft connected to a gearbox and the gearbox is connected to a power generator. When the shaft rotates, air is drawn through the intake duct to the compressor inlet, pressurized by the compressor, mixed with fuel in the combustor and ignited so that hot pressurized gas is formed. This gas expands across the turbine and leaves the APU through the gas exhaust. When expanding, the pressurized hot gas causes the turbine, shaft, and compressor to rotate.
The length, width and shape of an intake conduct is generally subject to position of the air intake and the position of the APU, and to the structural elements, components and/or aggregates that may be located between the air intake and the plenum entry of the APU. Thus, the air intake is normally be located in a forward position with respect to the APU so as to allow a stream of air to be guided to the APU in the smoothest possible manner. The duct air inlet is normally in connection with housing with an actuator flap used to close and open the air intake.
The APU plenum entry may be located in a position being lower or higher than the air intake or at the same level as the air intake. Thus, when the air intake is in higher position with respect to the APU, the first bend portion of the intake duct may extend downwards from the straight portion and connect with the second bend portion, whilst the second bend portion of the intake duct may extend leftwards or rightwards from the first bend portion. Also, when the air intake is in a lower position with respect to the APU, the first bend portion of the intake duct may extend upwards from the straight portion and connect with the second bend portion, whilst the second bend portion of the intake duct may extend leftwards or rightwards from the first bend portion.
APUs generate high levels of noise at the outside of the aircraft. This is especially unpleasant and even harmful when the APU is operating to power the aircraft systems prior to take off. An intake duct for the APU therefore must not only be suitable to efficiently provide a uniform flow of air to the APU but should also contribute to reduce noise propagation through the APU intake. For this purpose, it is known to place aeroacoustic guide vanes within an intake duct having a bend in a position following the curvature of the bend. Whilst this has proven to provide a rather efficient air-flow control and reduction of noise propagation, it is not fully satisfactory in respect of intake ducts having two bend portions, as for example in the case where the air intake is in an upper position with regard to the plenum inlet and where thus two internal bend portions are required on its path to the APU. Taking into account that the presence of the two internal bends leads to high values of flow distortion measured by the distortion coefficient in the plenum entry with the conventional guide vanes definition, conventionally placed guide vanes have proven to be unsatisfactory in intake ducts with two bended portions.